In recent years, a semiconductor laser for outputting a laser light is developing rapidly and, also, an optical transmission rate is increasing rapidly. At the same time, in a region between relatively high bit-rates, in which the optical transmission rate becomes a range form 125 Mbit/s (megabits/second) to 40 G (giga) or more bits/s, the optical pulse is increasingly used in a variety of optical communication technologies. Particularly, an increase in a communication using a high bit-rate of 40 G or more bits/s in the future is expected. In the high bit-rate region, an optical pulse width becomes a ps (pico second 10−12) or shorter order.
As a rule, the optical fiber for sending the optical pulse produces group velocity dispersion, self phase modulation, and polarization mode dispersion to cause the waveform deterioration of the optical pulse. A higher transmission rate of the optical pulse causes a short pulse width resulting in a serious effect of waveform deterioration of the optical pulse on a signal processing.
The optical pulse evaluation device can evaluate a form of an output waveform and the presence and absence of jitter for the optical pulse having a variety of bit-rates or the optical pulse of the high bit-rate, which are output from an optical pulse generating light source. On the other hand, for a variety of optical components and optical devices such as the optical fiber used in an optical network transmission system, the optical pulse evaluation device evaluates wavelength dispersion, polarization mode dispersion, higher dispersion, and other characteristic of the optical pulse to contribute to developments of optical communication systems and developments of optical components.
On the other hand, conventionally, the optical pulse evaluation device widely uses an approach for evaluation of the optical pulse by using a nonlinear optical effect. The nonlinear optical effect is a phenomenon in which a relation between an electromagnetic field of a laser light and electronic polarization of a matter becomes nonlinear. Methods for evaluation of the optical pulse by using the nonlinear optical effect include (1) auto correlation method, (2) cross correlation method, (3) FROF (frequency resolved optical gating) method, and (4) SPIDER (spectral phase interferometry for direct electric field reconstruction) method.
According to a first proposal of approaches using such the nonlinear optical effect, a measured optical pulse is distinguished from the optical pulse produced by four-wave mixing to eliminate a noise caused by an interference of the measured optical pulse and the optical pulse produced by four-wave mixing. In this way, the optical pulse is measured in a high sensitivity and, also, a spectrogram is divided into two orthogonal light components to evaluate the optical pulse produced by polarized wave dispersion (see, for example, patent document 1.) According to this first proposal, the measured optical pulse is divided into a probe light and a gate light, and a two photon absorption intensity is measured as a function of a delay time and a frequency by using a two photon transition medium, which is one of nonlinear effects.
On the other hand, according to a second proposal, a second harmonic wave of the measured optical pulse is generated by using a nonlinear optical material and, at the same time, a cross correlation signal light, which corresponds to a different frequency of the measured optical light from the second harmonic wave, is generated to convert this cross correlation signal light to an electric signal for displaying its pulse waveform (see, for example, patent document 2.)
Moreover, according to a third proposal, the measured optical pulse is divided into 2 optical pulses, and these 2 optical pulses are launched into a nonlinear optical material by making a delay time and a pulse width and a pulse waveform of the optical pulse are measured by using an optical pulse intensity waveform, which is a correlation data with the delay time, of the second harmonic wave (see, for example, patent document 3 and patent document 4.)
FIG. 1 shows a configuration of the optical pulse evaluation device according to this third proposal. This figure expresses the outline of the device disclosed in patent document 3. Measured light 11 is launched into interferometer 12 to go to an auto correlation signal detection unit 13. Interferometer 12 divides measured light 11 into 2 optical pulses 16 and 17 by using first and second parallel flat plates 14 and 15 to make an optical path difference between them. This optical path difference can be changed by driving unit 18 housed in interferometer 12. Auto correlation signal detection unit 13 measures a time correlation between 2 optical pulses 16 and 17 launched from interferometer 12 to calculate the optical pulse width of measured light 11.
Patent document 4 in the third proposal is same as patent document 3 in the point that measured light 11 is divided to delay the one optical pulse, and then, the correlated signals are measured after convoluting. However, in this document, a wavelength of the optical pulse is dispersed before measured light 11 is launched. In conclusion, patent document 4 uses an approach of measuring the optical pulse width by applying the auto correlation method and the nonlinear optical effect.
According to patent document 4, the optical path difference between 2 light paths is made by using a Mach-Zehnder interferometer, these light paths are converted to the electric signals by a light receiver, and a signal processing is carried out for each spectrum by using a frequency intensity analyzer to obtain an auto correlation (see, for example, patent document 5.)
FIG. 2 shows the configuration of the optical pulse evaluation device according to the fourth proposal. The light emitted from light source 21 is divided by first half mirror 23. The light passed through first half mirror 23 produces the predetermined frequency difference by frequency converter 24 and passes through second half mirror 25 to be input to light receiver 26. The light reflected by first half mirror 23 is delayed by photodelay device 27 followed by reflection by second half mirror 25 to be input in light receiver 26. Light receiver 26 receives 2 lights, of which frequencies and arrival times differ from each other, to convert to the electric signals finally followed by subjecting to the signal processing for each spectrum by using frequency intensity analyzer 28 located in its output end.
Patent document 1: Japanese Published Unexamined Patent Application No. 2003-28724 (paragraph 0041, FIG. 21)
Patent document 2: Japanese Published Unexamined Patent Application No. 2002-257633 (paragraph 0025, FIG. 2)
Patent document 3: Japanese Published Unexamined Patent Application No. 2001-74560 (paragraph 0081, FIG. 2)
Patent document 4: Japanese Published Unexamined Patent Application No. 2000-356555 (paragraph 0010, FIG. 1)
Patent document 5: Japanese Published Unexamined Patent Application No. 1997-133585 (paragraph 0207, FIG. 1)
In general, carrying out a characteristics evaluation of the pulse width and the pulse waveform of the optical pulse by using the nonlinear optical effect creates the problem in that the sensitivity of the measurement is low and accuracy of measurement is not significantly improved. According to the first proposal, the optical pulse is evaluated by applying the nonlinear optical effect and, thus, accuracy of measurement is limited depending on intensity of an incident electric field. Therefore, the sensitivity of measurement cannot be increased to disturb the characteristics evaluation of the optical pulse in use of the light source having a low power. On the other hand, this proposal creates the problem in that when evaluation of the optical pulse used for high transmission velocity with a high bit-rate is carried out, a high time resolution performance such as pico second or shorter is required as the pulse width of the optical pulse to be evaluated becomes short.
According to the second proposal, the second harmonic wave of the sampling light is generated and, at the same time, a material called pseudo phase matching element is necessarily used through a special procedure for generating a difference frequency light as a cross correlation signal light between the generated second harmonic wave and the measured light. In other words, the second proposal has a constraint for commercialization, if no nonlinear optical material is available to satisfy a phase matching condition for the wavelength of the measured light. On the other hand, this proposal uses also the nonlinear optical effect for evaluation of the optical pulse. Therefore, the sensitivity of the measurement is low to make the characteristics evaluation of the optical pulse inappropriate in use of the light source providing the low power.
The third proposal is same as the first and the second proposals in the point of using the nonlinear optical effect. Hence, measurement depends on the incident electric field to make the sensitivity of the measurement low and, thus, to make the characteristics evaluation of the optical pulse inappropriate in use of the light source providing the low power.
According to the fourth proposal, phase information becomes available by measuring the auto correlation of electric field components of the measured optical pulse and, thus, the auto correlation method is realized allowing a high relative sensitivity and almost no limitation of a measurable wavelength range in a low light intensity. However, according to the fourth proposal, the circuit configuration and a circuit control of frequency intensity analyzer 28 shown in FIG. 2 is complicated and, hence, the measurement can be unstably carried out for a position and a frequency drift of the optical pulse. Therefore, the proposal is inappropriate for evaluating the optical pulse for high velocity transmission, which has a relatively high bit-rate and a small pulse width, highly desired for the characteristics evaluation.
On the other hand, any optical pulse evaluation device cannot observe a spectral intensity to each spectral phase in a region of the optical pulse, which has a relatively high bit-rate, namely, cannot observe an actual pulse waveform.
The spectral phase will now be additionally described below. When a complex notation of an electric field spectrum of the optical pulse is denoted by E(ω), this can be expressed by the following formula:E(ω)=|E(ω)|exp[iφ(ω)]  (1)As known from this formula, the complex notation E(ω) of the electric field spectrum of the pulse can be expressed by a amplitude |E(ω)| and an argument φ(ω). This argument φ(ω) is named the spectral phase.
Consequently, in the above described devices conventionally proposed, the intensity of the optical pulse and the pulse width based on a delay processing to be evaluated are obtained. In such conventional optical pulse evaluation devices, the actual pulse waveform of the optical pulse is presumed by applying information obtained about the optical pulse to the form of a standard pulse waveform (for example, an eight and a width of a Gaussian waveform). Such the characteristics evaluation does not allow showing an actual distortion of a waveform which makes high accuracy evaluation impossible for a deterioration behavior of the waveform of the optical device itself or the waveform caused by the optical device such as the optical fiber.
So far, the deterioration behavior of the waveform of the optical device itself or the waveform, which are caused by the optical device such as the optical fiber, cannot be precisely evaluated in a state, where a data communication is operated.